Poly(organophosphazenes) constitute a substantial new class of elastomers, thermoplastics and fibers which are beginning to find use in a few applications and show potential in a variety of uses. The poly(organophosphazenes) are prepared by substitution of the halogen atoms of a polymeric intermediate, poly(dihalophosphazene), with organic moieties.
This invention relates to the preparation of poly(dihalophosphazenes) which are soluble in various inert solvents including chlorobenzenes, benzene, toluene, tetrahydrofuran and mixtures of such solvents, and the poly(aryloxyphosphazenes) resulting therefrom in solution or bulk polymerization.
More specifically, it relates to the polymerization of dichlorophosphazene of the formula (Cl.sub.2 PN).sub.n wherein n is 3 to 7 to polymers of the formula (Cl.sub.2 PN).sub.20-50,000 wherein the polymerization is accomplished in the presence of a sulfamic acid compound and derivatives thereof as a catalyst. It relates to polymers and copolymers therefrom of poly(aryloxyphosphazenes).
The uncatalyzed bulk polymerization of (Cl.sub.2 PN).sub.3, (Cl.sub.2 PN).sub.4 or mixtures thereof to form soluble (Cl.sub.2 PN).sub.n polymer is described in U.S. Pat. No. 3,370,020. The compounds are polymerized by heating them at a temperature and for a length of time ranging from about 200.degree. C for 48 hours to 300.degree. C. for 30 minutes.
Another known process for producing soluble poly(dichlorophosphazenes) is described by Allcock and Kugel (J. Am. Chem. Soc., 87, 4216 (1965)) and elsewhere (U.S. Pat. No. 3,515,688 in Example 1) wherein cyclic trimeric (Cl.sub.2 PN).sub.3 is heated in an evacuated sealed tube for stated periods up to 48 hours at polymerization temperatures.
It has been long known that strong acids also catalyze the polymerization of (Cl.sub.2 PN).sub.2 both in bulk and in solution. Even acid washed tubes used in the polymerization have catalyzed the reaction. Also, oxidizing agents such as oxygen, sulfur and acetone as well as water have been found to catalyze the reaction. Many of these catalysts produce low molecular weight polymer and, at the same time, produce highly cross-linked polymer in bulk polymerizations.
Catalytic preparation of soluble poly(dichlorophosphazenes) is described in U.S. Pat. No. 4,005,171 wherein the catalyzed polymerization is conducted in solution or in bulk at temperatures of from about 130.degree. C. to 220.degree. C. for periods ranging from one hour to several days. The catalysts are selected from the group consisting of certain strong acids such as sulfonic acids, the metal or organo-metal salts of certain strong acids, certain polyhalocarboxylic acids, salts of certain miscellaneous acids and certain substituted cyclophosphazenes. Not all strong acids and their salts are active catalysts for the polymerization of cyclic (Cl.sub.2 PN).sub.3 to (Cl.sub.2 PN).sub.n polymer. According to U.S. Pat. No. 4,005,171, HCl, HBr, HgCl.sub.2 and HgBr.sub.2 are not catalysts when used alone.
Catalytic conversion of low molecular weight cyclochlorophosphazenes to high molecular weight linear polydichlorophosphazenes is described in U.S. Pat. No. 4,123,503 and in German Patent Offenlegungsschrift No. 26 37 534. The thermal polymerization of (NPCl.sub.2).sub.n as is described in U.S. Pat. Nos. 3,370,020 and 3,515,688 and other sources already mentioned are disclosed in '503 and 26 37 534 as carried out at lower temperatures and with improved rates of conversion by using Lewis-acid compounds of aluminum and bromine. Molecular weight of the product is controlled by variation of the concentrations of the catalyst. A certain control of the molecular weight distribution of the polymerizate obtained is possible by supplementary addition of the catalyst and/or cyclic oligomers.
U.S. Pat. No. 3,937,790 discloses the use of small amounts of water as a catalyst for the polymerization of chlorocyclophosphazenes. Very low concentrations of water within the ranges of from about 0.005 mol percent to 0.10 mol percent gave (NPCl.sub.2).sub.3 polymers with a desired intrinsic viscosity but when more than 0.2 mol percent of water was present the molecular weight of the resulting polymer was too low for elastomer applications and above 0.3 mol percent, yield of polymer decreased and crosslinked polymer was formed.
Poly(dichlorophosphazene) derived either by thermal polymerization or catalytic polymerization are precursors for poly(aryloxyphosphazenes). Polydichlorophosphazene is hydrolytically unstable. Replacement of the chloride with aryloxide substituents to yield fully substituted linear polyaryloxyphosphazenes results in a hydrolytically stable polymer with characteristically high molecular weights (M.sub.w .about.10.sup.6) and broad molecular weight distributions (M.sub.w /M.sub.n &gt;10). M.sub.w is defined as weight average molecular weight, M.sub.n is defined as number average molecular weight. M.sub.w /M.sub.n is defined as the ratio of the weight average molecular weight to the number average molecular weight range and is termed molecular weight distribution or polydispersity. Mathematically, M.sub.n =.SIGMA.N.sub.x M.sub.x wherein N.sub.x is the mole fraction of molecules of size M.sub.x .multidot.M.sub.w =.SIGMA.W.sub.x M.sub.x wherein W.sub.x is the weight fraction of molecules of size M.sub.x.
Poly(organophosphazenes) in the prior art characteristically have broad bimodal molecular weight distributions (MWD). Generally M.sub.w /M.sub.n is greater than 10. It has been found that M.sub.w /M.sub.n is greater than 10 especially if the intermediate, poly(dichlorophophazene) is prepared via an initiated solution polymerization.
As is discussed by R. E. Singler and G. L. Hagnauer (Organometallic Polymers, Academic Press, New York, 1978, p. 257-269) the macromolecular characteristics of polyphosphazenes are of a broad distribution with high molecular weight tails which Singler and Hagnauer indicate as depending on polymerization and substitution reaction conditions as well as the functionality of the polymer chain side group.
A narrow molecular weight distribution is often preferred over a broad molecular weight distribution because of the accompanying physical characteristics of the polymer. A narrow molecular weight distribution can be preferable for applications wherein greater crystallinity is required. Conversely, a broad molecular weight distribution is preferred wherein less crystallinity is required by the application. The ability to tailor molecular weight distribution (including modality) and molecular weight of poly(organophosphazenes) accordingly is of economic value.
It is considered that monomadality of the molecular weight range tends to give a better rate of crystallization of the polymer. The cause of bimodal distribution of the broad molecular weight range of poly(organophosphazenes) has been theorized (G. L. Hagnauer and B. R. La Liberte, Jo. Ap. Poly. Sci., 20, 3086 (1976)) as due to heterogeneities in the bulk polymerization step, impurities in the cyclic trimer or surface catalysis effects as well as reaction conditions in the synthesis procedure of the intermediate, the poly(dichlorophosphazene). Fractionation has succeeded in narrowing the broad molecular distribution (G. L. Hagnauer and B. R. La Liberte, Jo. Polymer Sci. 14, 371 (1976)); however bimodal distribution has remained a problem.
The primary object of this invention is to provide a simple process for the preparation of poly(organophosphazenes) wherein control of molecular weight distribution or polydispersity is obtained.
Another object of this invention is to provide a process for preparation of poly(organophosphazenes) wherein a narrow molecular weight distribution with monomodality is obtained. Another object of this invention is to provide a catalyst which exhibits improved activity for the solution or bulk polymerization of halocyclophosphazenes.
These and other objects and advantages of the present invention will become clear from the following specification. These objects have been attained in accordance with the present invention using sulfamic acid compounds as catalysts.
For example, it has been found that sulfamic acid which is similar in strength to HCl and HBr catalyzes the polymerization of dichlorophosphazene wherein the polymerization is accomplished in solution or bulk, even though sulfamic acid is insoluble in the reaction medium, being present in the form of a suspension. The poly(dichlorophosphazenes) obtained therefrom can be precursors for poly(aryloxyphosphazenes) wherein the molecular weight range surprisingly can be controlled to within a narrow molecular weight range where M.sub.w /M.sub.n is below 10 or has a broad molecular weight range wherein M.sub.w /M.sub.n is above 10. Sulfamic acid has the additional advantage that the polymerizate potentially can be monomodal.